1. Field of the Invention
The present invention relates generally to a miniature piezoelectric motor, and in particular to a rotary piezoelectric motor and a linear piezoelectric motor and method of exciting an ultrasonic traveling wave for driving the motor.
2. Description of the Related Art
Piezoelectric ultrasonic motors with their exceptional properties, such as high resolution of displacement control, absence of parasitic magnetic fields, frictional locking at the power-off stage, and high thrust to weight ratio, make them good candidates for use in precision micromechanical systems.
Piezoelectric motors have several advantages over a conventional electromagnetic motor. These include a faster response time, a high power-to-weight ratio, and smaller packaging capability. They also have several disadvantages, including the need for high voltage, high frequency power sources, and potential wear at the rotor/stator interface. These motors operate using a ferroelectric ceramic element to excite ultrasonic vibrations in a stator structure. The elliptical movement of the stator is converted into the motion of a sliding plate in frictional contact with the stator. The resulting movement is either rotational or linear, depending on the design of the structure.
Conventional piezoelectric ultrasonic motors can be mainly classified into two classes: 1) traveling wave ultrasonic motors, and 2) standing wave ultrasonic motors. A conventional disc-type or ring-type traveling wave motor (rotary motor) is made from a piezoelectric disc (or ring) and a metal disc. With a piezoelectric d31 effect, the piezoelectric disc (or ring) and metal disc composite stator produces a flexure-flexure mode traveling wave that drives a contact rotor through a frictional force. The most frequently researched operating principle for a linear piezoelectric motor is based on the excitation of a longitudinal and a superimposed bending mode of a rectangular piezoelectric plate, to achieve the elliptic motion of the driving tip.
A disc-type traveling wave rotary motor, such as that developed by Matsushita Electric Company Ltd., is representative of a traveling wave type of motor. The piezoelectric stator for this type of motor includes a composite disc comprised of a metal elastic disc and two piezoelectric ceramic discs with thickness polarization. A traveling flexure vibration mode is excited by each section of the piezoelectric discs, and produces a transverse width extension mode with d31 effect under two ac voltages.
Another typical configuration for the conventional traveling wave ultrasonic rotary piezoelectric motor is a ring-type ultrasonic motor, such as that developed by Sashida. This motor utilizes the axial bending-vibration mode of a circular ring with d31 effect of the piezoelectric ceramic ring. The piezoelectric ring-type element is polarized in the thickness direction and produces a transverse length extension mode with d31 effect under ac voltages.
A typical configuration of the conventional linear ultrasonic motor, such as that developed by Yoshiro Tomikawa, is driven by the elliptic motion of the combination displacement of longitudinal (d31) and secondary bending modes. The motor operates according to the principle that at a certain distance to length ratio of a rectangular-shaped piezoelectric ceramic plate, the resonant frequencies of first longitudinal and second bending modes coincide with each other. The elliptic motion is obtained by the combination of the two vibrations.
While all the above described rotary and linear motors offer satisfactory performance, they primarily utilize a transverse length extension mode for exciting a traveling flexure wave with a low d31 piezoelectric effect and low k31 electromechanical coupling effect. In the operational mode, the relatively low d31 and k31 effects of the piezoelectric ceramic material hinders additional development of these types of motors.
Thus, there is a need in the art for a system and method of providing a rotary motor which utilizes a rotary shear vibration mode and a linear ultrasonic motor which uses the linear shear vibration mode, respectively, of piezoelectric ceramics.